Can an employer agree with an employee that they will retire at a particular age?
Although an employer cannot dismiss an employee on grounds of retirement (unless this is objectively justified), it is not unlawful for it to agree that they will retire at a particular age if the employee wishes to do so. The employee is essentially resigning by reason of their wish to retire. A retirement will be lawful if it is the employee's decision and no pressure has been placed on them to retire.
However, if the employee changes their mind about retiring at a particular age, before giving notice of the resignation, the employer will not be able to enforce the agreement and insist that the employee retire.